One method for attaching an integrated circuit onto a substrate in semiconductor packaging operations is the so-called flip-chip technology method. In flip-chip technology, the active side of the semiconductor die is bumped with metallic solder balls and flipped so that the solder balls can be aligned and placed in contact with corresponding electrical terminals on the substrate. Electrical connection is realized when the solder is reflowed to form metallurgical joints with the substrates. The coefficients of thermal expansion (CTE) of the semiconductor die, solder, and substrate are dissimilar and this mismatch stresses the solder joints, which ultimately can lead to failure of the semiconductor package.
Organic materials, often filled with organic or inorganic fillers or spacers, are used to underfill the gap between the die and the substrate to offset the CTE mismatch and to provide enforcement to the solder joints. Such underfill materials can be applied through a capillary effect, by dispensing the material along the edges of the die-substrate assembly after solder reflow and letting the material flow into the gap between the die and substrate. The underfill is then cured, typically by the application of heat.
In an alternative process known as pre-applied, an underfill material is applied onto a solder bumped semiconductor wafer, through printing if the material is a paste, or through lamination if the material is a film. The wafer is singulated into dies and an individual die subsequently bonded onto the substrate during solder reflow, typically with the assistance of temperature and pressure. This process can accomplish solder reflow and underfill cure in one operation, or the underfill can be cured in an additional process step following reflow.
In another process known as no-flow, an underfill material is applied onto the substrate, a flip-chip is placed on top of the underfill, and, typically with the assistance of temperature and pressure, the solder is reflowed to realize the interconnection between the die and substrate. Such reflow process is accomplished on thermal compression bonding equipment, in a time scale that can be as short as a few seconds. The underfill material can be cured under these same conditions, or, as with the pre-applied, in an additional process step.
In all three of these underfill operations, the solder must be fluxed either before or during the reflow operation to remove any metal oxides present, as the presence of metal oxides hinders reflow of the solder, wetting of the substrate by the solder, and electrical connection. For capillary flow operations, fluxing and removal of flux residues is conducted before the addition of the capillary flow underfill. For the no-flow and pre-applied underfill operations, the fluxing agent is added to the underfill material and fluxing occurs during reflow of the solder.
Commonly, the fluxing agents in no-flow underfill materials are typically organic acids or anhydrides, and are not suitable for chemistries that are sensitive to acidic species, such as, cyanate ester based underfill resins. In such chemistries, the acidic fluxing agents shorten underfill shelf-life due to premature gelation, and they can corrode solder interconnects in the cured underfill material.
As with any noncrystalline polymer or ceramic, cured underfill compositions have two coefficients of thermal expansion (CTE). Below the glass transition temperature (Tg), the CTE is commonly referred to as α1. Above the glass transition temperature, the CTE is commonly referred to as α2. Typically, α2 is much higher than α1. An underfill with a high Tg will remain in α1 throughout the range of operating temperatures experienced; an underfill with a low Tg will more likely enter α2 during normal use conditions, causing excessive expansion and contraction. This could lead to solder joint and device failure.
Additionally, it is known that the modulus of underfill drops quickly above the Tg; therefore, underfill compositions with higher Tg values provide better support to solder joints during any temperature cycling that occurs during fabrication or operation. For smaller semiconductor chips and for relatively soft solder, such as high lead alloys, having a high Tg is not as critical as it is for larger chips and lead free solders. However, the current practice in the flip-chip industry is to move to larger chips and lead free solder, thus creating a need for underfill with high Tg values, preferably over 100° C. and even over 130° C.